1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly, to an injection molding machine which determines a closing state of a check ring based on a variation of a resin pressure when a screw is rotated by a constant amount to carry out controlling of closing operation of a check ring.
2. Description of the Related Art
A screw of an inline injection molding machine is provided with a check ring. There are various kinds of check rings and as a representative check ring, there is a known check ring in which a pressure in front of the check ring is relatively higher than a pressure behind the check ring, and a pressure difference is generated, thereby closing the valve.
It is known in Japanese Patent Application Laid-open No. 62-060621 that a screw is rotated in reverse (in a direction opposite from the rotation direction of the screw in a metering step) simultaneously when injection is started, and the check ring is closed.
It is known in Japanese Patent Applications Laid-open Nos. 3-45325, 4-71817 and 9-174629 that if the screw is normally rotated, a first resin flow path and a second resin flow path of the check ring are brought into communication with each other, and if the screw is rotated in reverse, the communication between the first resin flow path and the second resin flow path of the check ring is cut and they are sealed.
It is known in Japanese Examined Patent Publication No. 47-8380, and Japanese Patent Applications Laid-open Nos. 60-201921 and 62-19423 that if the screw is rotated in reverse, the check ring is relatively retreated from the screw to seal the resin flow path.
These check rings have a mechanism which opens and closes a flow path when the check ring rotates by a predetermined amount with respect to a screw head. In the injection molding machine having such a check ring, control is performed such that if the screw is rotated in reverse by a predetermined amount after the metering operation is completed, thereby closing the flow path and then, injection is carried out. A set value of the reverse rotation amount is calculated and set from a distance between the check ring when it is opened and a check seat.
When the screw rotates the screw in reverse head rotates in reverse at the same speed, and the check ring also rotates in reverse together with the screw head at a given speed. When the screw rotates by a predetermined amount, the rotation amount of the check ring with respect to the screw head is smaller than the reverse rotation amount of the screw. Therefore, when the screw is rotated in reverse to close the flow path, reverse rotation of the screw greater than reverse rotation obtained from a design value of a closing mechanism of the check ring is required.
A reverse rotation amount of the screw which is necessary to close the check ring is varied depending upon a viscosity of resin and the injection speed. Therefore, even if the calculated reverse rotation amount is set, the reverse rotation amount is insufficient and the check ring can not be closed perfectly or the reverse rotation amount becomes excessively large in some cases.
When the reverse rotation amount is insufficient and the flow path is not closed, there is inconvenience that resin reversely flows rearward of the screw during injection. When the flow path is closed but the reverse rotation amount is excessively large, resin reversely flows backward of the screw more than necessary during the reverse rotation, and there is inconvenience that the metering time is increased and resin resides too long in the cylinder.